PostgreSQL 9.1 includes proper extension support, as you might well know if you ever read this very blog here. Some hosting facilities are playing with PostgreSQL at big scale (hello Heroku!) and still meet with small caveats making their life uneasy.
To be specific, only superusers are allowed to install C coded stored procedures, and that impacts a lot of very useful PostgreSQL extension: all those shiped in the contrib package are coded in C.
